Distribution of sedimentary rocks in space and time

Distribution of sedimentary rocks in space and time

1. Distribution of sedimentary rocks in space and time
1
Sedimentary rocks and sediments range in age from
Precambrian to modern. The ages of the
oldest known sedimentary rocks (in Greenland and
northern Quebec, Canada) have been determined by iron
isotope analyses to be about 3.7–3.8 billion years.
The first rocks that formed on Earth were probably basic
volcanic rocks. Sedimentary rocks began to form once
Earth’s atmosphere and oceans had developed owing to
degassing of Earth’s interior.

2. 2
Methods for studying sedimentary
structures
Sedimentary structures provide important
information about depositional conditions and
depositional environments. Also, many sedimentary
structures have directional significance, making
them useful indicators of paleo-current direction.
Analysis of sedimentary structures thus plays a
crucial role in basin analysis. Basin analysis is a
broad term that encompasses all of the various
methods of study (geophysical, geochemical,
sedimentologic, stratigraphic) that are focused on
the evaluation of depositional systems.

3. Methods for studying sedimentary structures
3
Information gained from such evaluation may
have economic significance in petroleum and
mineral exploration and significance to
interpretation of geologic and tectonic history,
including plate reconstructions, regional
historical geology, and
paleogeography.
Because study of sedimentary structures
constitutes such an important part of
environmental interpretation and basin analysis,
much work has gone into developing suitable
methods for their study.

4. 4
Most study of sedimentary structures takes
place in the field where structures are exposed
in outcrop and can be studied and
photographed.
Replicas of structures, called peels, may also be
made in the field and taken back to the
laboratory for further study.
Also, samples of sediment buried on land and in
subaqueous environments can be obtained by
suitable coring devices, such as box corers, for
subsequent laboratory study.
X-Radiography techniques are particularly useful
for revealing faint structures in such cores.
Methods for studying sedimentary
structures

5. 5
Paleocurrent analysis
Determining paleoflow (paleocurrent) directions is
an important aspect of much field study.
Directional sedimentary structures, such as
cross-bedding and ripple marks, provide reliable
information for determining paleocurrent
directions.
The orientation of directional sedimentary
structures (e.g. the dip direction of cross-bed
foresets) is determined in the field with a Brunton
compass by taking measurements from as many
different outcrops as practical. If directional
structures have been tilted owing to tectonic uplift
of their host beds, a correction must be made for
this tilt

6. 6
Directional data must, therefore, be treated
statistically to reveal primary and secondary
paleocurrent trends. Depositing paleocurrents
may have flowed in a single direction
(unidirectional), two directions (bidirectional), or
three or more directions
(polydirectional).
Corresponding paleocurrent data are thus said to
be unimodal (e.g. river sediment), bimodal (e.g.
tidal sediment), or polymodal (e.g. shoreline
sediment).
Geologists commonly plot paleocurrent data on
so-called rose diagrams to reveal the paleocurrent
direction(s)
Paleocurrent analysis

7. Questions
7
1. What are the deposition of a sedimentary rock?
2. How do you identify sedimentary structures?
3. What are the different methods of
sedimentary rock?
4. What are the 4 types of sedimentary
structures?
5. How can sedimentary rocks be used to
determine paleoenvironments?